Fabrication of highly textured lithium cobalt oxide films by rapid thermal annealing

ABSTRACT

Systems and methods are described for fabrication of highly textured lithium cobalt oxide films by rapid thermal annealing. A method of forming a lithium cobalt oxide film includes depositing a film of lithium cobalt oxide on a substrate; rapidly heating the film of lithium cobalt oxide to a target temperature; and maintaining the film of lithium cobalt oxide at the target temperature for a target annealing time of at most, approximately 60 minutes. The systems and methods provide advantages because they require less time to implement and are, therefore less costly than previous techniques.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY-SPONSOREDRESEARCH AND DEVELOPMENT

This invention was made with Government support under contract No.DE-AC05-960R22464 awarded by the United States Department of Energy toLockheed Martin Energy Research Corp., and the Government has certainrights in this invention.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates generally to the field of lithium cobalt oxide(LiCoO₂) films. More particularly, the invention relates to fabricationof highly textured LiCoO₂ films by rapid thermal annealing.

2. Discussion of the Related Art

Thin films of LiCoO₂ are used as the cathode in thin film lithium andlithium ion batteries. The films are deposited by rf magnetronsputtering of LiCoO₂. FIG. 1 shows an x-ray diffraction pattern of anLiCoO₂ film annealed for two hours at 700° C. according to the priorart. In order to crystallize the films and achieve the high degree ofpreferred orientation that optimizes battery performance, they areheated at 700° C. in flowing O₂ for 2 hours. In order to lower the costof manufacturing, it is desirable to shorten the annealing time as muchas possible without compromising battery performance.

Heretofore, the requirements of low cost manufacturing and shortannealing times referred to above have not been fully met. What isneeded is a solution that addresses each of these requirementsindependently or concurrently. The invention is directed to meetingthese requirements, among others.

SUMMARY OF THE INVENTION

A goal of the invention is to reduce the costs associated with LiCoO₂thin films. Another goal is to reduce the annealing times associatedwith the manufacture of LiCoO₂ thin films.

One embodiment of the invention is related to a method for manufacturingLiCoO₂ films. The method includes depositing the LiCoO₂ film, rapidlyheating the LiCoO₂ film to a target temperature and holding thetemperature of the LiCoO₂ film at the target temperature for a targetannealing time.

The method can also include rapid cooling of the LiCoO₂ film afterholding the LiCoO₂ film within the target temperature range for thetarget annealing time. The target temperature is preferably greater thanapproximately 700° C., more preferably at least approximately 800° C.and most preferably at least approximately 850° C. Further, the targettemperature range is preferably approximately 700-1000° C., morepreferably approximately 750-950° C. and most preferably approximately850-950° C. The target annealing time is preferably at mostapproximately an hour, more preferably at most approximately 30 minutesand most preferably at most approximately 15 minutes. Further, thetarget annealing time is preferably approximately 5-60 minutes, morepreferably approximately 10-20 minutes and most preferably about 12-17minutes.

The invention includes cathodes made according to the above methods. Theinvention also includes batteries which include these cathodes.

These, and other, goals and embodiments of the invention will be betterappreciated and understood when considered in conjunction with thefollowing description and the accompanying drawings. It should beunderstood, however, that the following description, while indicatingpreferred embodiments of the invention and numerous specific detailsthereof, is given by way of illustration and not of limitation. Manychanges and modifications may be made within the scope of the inventionwithout departing from the spirit thereof, and the invention includesall such modifications.

BRIEF DESCRIPTION OF THE DRAWINGS

A clear conception of the advantages and features constituting theinvention, and of the components and operation of model systems providedwith the invention, will become more readily apparent by referring tothe exemplary, and therefore nonlimiting, embodiments illustrated in thedrawings accompanying and forming a part of this specification. Itshould be noted that the features illustrated in the drawings are notnecessarily drawn to scale.

FIG. 1 illustrates an x-ray diffraction pattern of a LiCoO₂ filmprepared according to the prior art.

FIG. 2 illustrates an x-ray diffraction pattern of a LiCoO₂ filmprepared by rapidly heating a LiCoO₂ film to 900° C. for 15 minutesrepresenting an embodiment of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention and the various features and advantageous details thereofare explained more fully with reference to the nonlimiting embodimentsthat are illustrated in the accompanying drawings and detailed in thefollowing description of preferred embodiments. Descriptions of wellknown components and processing techniques are omitted so as not tounnecessarily obscure the invention in detail.

The below-referenced U.S. Patent disclose embodiments that weresatisfactory for the purposes for which they were intended. The entirecontents of U.S. Pat. Nos. 5,314,765; 5,338,625; 5,512,147; 5,561,004;5,569,520; 5,567,210; 5,597,660; and 5,612,152 are hereby expresslyincorporated by reference into the present application as if fully setforth herein.

The invention can include LiCoO₂ films and/or highly textured LiCoO₂cathodes. The concept of the invention can include the use of rapidthermal annealing to achieve high crystallinity and texturing in thesefilms and/or cathodes. Although these features are independent, they canbe combined in embodiments of the invention.

The invention can include a method for manufacturing LiCoO₂ films. Themethod includes depositing a LiCoO₂ film on a substrate, rapidly heatingthe LiCoO₂ film to a target temperature and holding the temperature ofthe LiCoO₂ film at about the target temperature for a target annealingtime.

A suitable substrate for depositing the LiCoO₂ film includes, but is notlimited to, ceramics (such as lumina), semiconductors (such as silicon),or metals (such as stainless steel). Suitable techniques for depositingthe LiCoO₂ film on the substrate include, but are not limited to, RFmagnetron sputtering of LiCoO₂ targets in Ar and O₂ gas mixtures. Thefilm is preferably deposited to a thickness of approximately 2.5-3 μm(micrometers), more preferably to a thickness of approximately 1 μm andmost preferably to a thickness of approximately 2.5-3 μm. The termapproximately, as used herein, is defined as at least close to a givenvalue (e.g., preferably within 10% of, more preferably within 1% of, andmost preferably within 0.1% of).

After the LiCoO₂ film is deposited, it is rapidly heated to the targettemperature. Suitable equipment for rapidly heating the LiCoO₂ filmincludes, but are not limited to a tube furnace, box furnace etc. Apreferred method for the rapid heating of the LiCoO₂ film includes theuse of high intensity quartz lamps. The rate of temperature ramp upduring the heating is preferably approximately 50-100° C./minute. Thetime required for the LiCoO₂ film to reach the target temperature ispreferably at most approximately 15 minutes.

Once the LiCoO₂ film has achieved the target temperature, thetemperature of the LiCoO₂ film is preferably maintained near the targettemperature for a target annealing time. The target temperature ispreferably greater than approximately 700° C., more preferably at least800° C. and most preferably at least about 850° C. Further, the targettemperature range is preferably approximately 700-1000° C., morepreferably approximately 750-950° C. and most preferably approximately850-950° C. The target annealing time is preferably at most an hour,more preferably at most approximately 30 minutes and most preferably atmost approximately 15 minutes. Further, the target annealing time ispreferably approximately 5-60 minutes, more preferably approximately10-20 minutes and most preferably approximately 12-17 minutes.

While the LiCoO₂ film is maintained at the target temperature, thetemperature of the LiCoO₂ film will vary over a range due to the natureof imperfect process controls. This range is preferably at mostapproximately +/−5° C. and more preferably at most approximately +/−1 C.

After the LiCoO₂ film is maintained at the target temperature, theLiCoO₂ film can be rapidly cooled to room temperature. Suitableequipment for the rapid cooling include controlled removal from furnace.The ramp down rate of the LiCoO₂ film temperature is preferablyapproximately 50-100° C./minute.

The LiCoO₂ film is preferably exposed to an O₂ atmosphere during therapid heating of the LiCoO₂ film, while it is maintained at the targettemperature and/or during the rapid cooling of the LiCoO₂ film. Theexposure to the O₂ atmosphere can be created by flowing O₂ over theLiCoO₂ film.

The above methods can provide LiCoO₂ films for use with the cathodes inbatteries such as thin film lithium and lithium ion batteries. Thesemethods can provide LiCoO₂ films having a high degree of orientation ofgrains and microcrystals within the films that are preferred for optimumbattery performance.

EXAMPLE

Specific embodiments of the invention will now be further described bythe following, nonlimiting examples which will serve to illustrate insome detail various features of significance. The examples are intendedmerely to facilitate an understanding of ways in which the invention maybe practiced and to further enable those of skill in the art to practicethe invention. Accordingly, the examples should not be construed aslimiting the scope of the invention.

Films of LiCoO₂ were deposited by rf magnetron sputtering of LiCoO₂targets in Ar+O₂ gas mixtures. After deposition, the films were heatedrapidly in the presence of oxygen to temperatures of between 700 and900° C., held at the maximum temperature for 15 minutes or less, andthen cooled rapidly to room temperature. For example, films of LiCoO₂that were inserted rapidly into a tube furnace at 900° C. in flowing O₂and held for 15 minutes displayed the same high degree crystallinity andpreferred orientation as those films annealed at 700° C. for two hoursor longer. FIG. 2 shows an x-ray diffraction pattern of another LiCoO₂film made according to the invention. Specifically, annealed for 15 min.at 900° C. The positions of the (003), (101), and (104) reflections ofthe films are indicated; the remaining peaks are due to the aluminasubstrates and platinum current collectors. As shown in FIGS. 1 and 2,the high intensity and narrow widths of the (101) and (104) reflectionsindicate optimum preferred orientation and high crystallinity of theLiCoO₂ grains in the film annealed at 900° C. for 15 min. as compared tofilms annealed according to the prior art, (i.e. annealed at 2 hours at700° C.).

Films of LiCoO₂ thicker than about 1 gm develop a texture during theannealing process in which the majority of the grains are oriented withtheir (101) and (104) planes parallel to the substrate. This preferredorientation is due to the tendency to minimize the volume strain energydeveloped in the films during annealing As the film thickness decreasesbelow 0.5 μm, the texture changes so that most of the grains areoriented with their (003) planes parallel to the substrate as a resultof the tendency to minimize surface energy. It is our expectation thatlithium transport through (003) oriented grains is nil but rapid enoughthrough the (101)-(104) grains. Therefore, in cathodes with a higherpercentage of (003)-oriented grains, lithium transport occurs primarilythrough the grain boundaries. Because cell resistance is dominated bythe electrolyte and electrolyte-cathode interface. there was nosignificant dependence of cell resistance on cathode thickness, andtherefore it was not possible to measure the bulk resistance of thecathodes and determine the relative grain and grain boundarycontributions. The most important process variable in achieving lowresistance cathode films is the deposition temperature, Deposition athigh temperatures leads to larger grains and increased void fraction.This reduces the contact area between the electrolyte and cathode aswell as the contact area between the grains resulting in higher cellresistance.

LiCoO₂ films manufactured according to the methods of the invention arecost effective because they requires less time to manufacture. As aresult, the methods of the invention reduce the costs associated withmanufacturing LiCoO₂ films, cathodes including these films and batteriesincluding these cathodes.

A practical application of the invention that has value within thetechnological arts is film rechargeable lithium and lithium ionbatteries. There are virtually innumerable uses for the invention, allof which need not be detailed here.

All the disclosed embodiments of the invention described herein can berealized and practiced without undue experimentation. Although the bestmode of carrying out the invention contemplated by the inventors isdisclosed above, practice of the invention is not limited thereto.Accordingly, it will be appreciated by those skilled in the art that theinvention may be practiced otherwise than as specifically describedherein.

For example, the individual components need not be formed in thedisclosed shapes, or assembled in the disclosed configuration, but couldbe provided in virtually any shape, and assembled in virtually anyconfiguration. Further, the individual components need not be fabricatedfrom the disclosed materials, but could be fabricated from virtually anysuitable materials. Further, although the LiCoO₂ films described hereincan be a physically separate module, it will be manifest that the LiCoO₂films may be integrated into the apparatus (e.g., electrode) with whichit is associated. Furthermore, all the disclosed elements and featuresof each disclosed embodiment can be combined with, or substituted for,the disclosed elements and features of every other disclosed embodimentexcept where such elements or features are mutually exclusive.

It will be manifest that various additions, modifications andrearrangements of the features of the invention may be made withoutdeviating from the spirit and scope of the underlying inventive concept.It is intended that the scope of the invention as defined by theappended claims and their equivalents cover all such additions,modifications, and rearrangements. The appended claims are not to beinterpreted as including means-plus-function limitations, unless such alimitation is explicitly recited in a given claim using the phrase“means-for.” Expedient embodiments of the invention are differentiatedby the appended subclaims.

What is claimed is:
 1. A method of forming a textured LiCoO₂ film,comprising: depositing a film of LiCoO₂ on a substrate; rapidly heatingthe film of LiCoO2 to a target temperature; maintaining the film ofLiCoO2 at the target temperature for a target annealing time of at mostapproximately 60 minutes to form the textured LiCoO2 film; and thenrapidly cooling the textured LiCoO2 film.
 2. The method of claim 1,wherein the substrate includes at least one material selected from thegroup consisting of ceramics, semiconductors, and metals.
 3. The methodof claim 1, wherein the target temperature is greater than approximately700° C.
 4. The method of claim 1, wherein the target temperature is atleast approximately 800° C.
 5. The method of claim 1, wherein the targettemperature is at least approximately 850° C.
 6. The method of claim 1,wherein the target annealing time is at most approximately 30 minutes.7. The method of claim 1, wherein the target annealing time isapproximately 12-17 minutes.
 8. The method of claim 1, whereindepositing the film of LiCoO₂ includes depositing the film to athickness of approximately 2.5-3 μm.
 9. The method of claim 1, whereinrapidly beating the film of LiCoO₂ includes heating the film at a rateof at least approximately 50-100°C./min.
 10. The method of claim 1,wherein rapidly heating the film of LiCoO₂ includes delivering heat froma quartz lamp.
 11. The method of claim 1, wherein rapidly cooling thetextured LiCoO₂ film includes controlled removal of the film of LiCoO₂from a furnace.
 12. The method of claim 1, wherein rapidly cooling thetextured LiCoO₂ film includes delivering a cooling jet to the texturedLiCoO₂ film.
 13. The method of claim 12, wherein rapidly cooling thetextured LiCoO₂ film includes cooling the textured LiCoO₂ film at a rateof approximately 50-100° C./min.
 14. The method of claim 1, whereinmaintaining the film of LiCoO₂ at the target temperature includesflowing O₂ over the film of LiCoO₂.
 15. The method of claim 1, whereinmaintaining the film of LiCoO₂ of thickness equal or greater than 1 μmat the target temperature for a target annealing time of at mostapproximately 60 minutes results in a majority of grains oriented withtheir (101) and (104) planes parallel to the substrate.
 16. A cathodemade by the method of claim
 1. 17. A battery including the cathode ofclaim
 16. 18. The method of claim 1, wherein the target annealing timeis approximately 15 minutes.
 19. A method of forming a film of LiCoO₂,comprising: depositing a film of LiCoO₂ on a substrate; rapidly heatingthe film of LiCoO2 to a target temperature; and maintaining the film ofLiCoO2 at the target temperature for a target annealing time of at mostapproximately 60 minutes, wherein maintaining the film of LiCoO₂ ofthickness equal or greater than 1 μm at the target temperature for atarget annealing time of at most approximately 60 minutes results in amajority of grains oriented with their (101) and (104) planes parallelto the substrate.